Method for preparing battery-grade lithium carbonate by recycling lepidolite lithium extraction mother liquor

Abstract

The invention relates to the technical field of lithium carbonate recovery, in particular to a method for preparing battery-grade lithium carbonate by recovering lepidolite lithium extraction mother liquor. The method comprises the following steps: evaporating and concentrating the lepidolite lithium extraction mother liquor, separating out mixed salt, and cooling to obtain low-temperature crystal mush; carrying out carbonization reaction and solid-liquid separation on the low-temperature crystal mush to obtain primary separation mother liquor; carrying out cooling sodium separation and solid-liquid separation on the primary separation mother liquor to obtain frozen liquid and sodium sulfate decahydrate, and carrying out heating pyrolysis and solid-liquid separation on the frozen liquid to obtain crude lithium carbonate and primary pyrolysis mother liquor; performing size mixing on the crude lithium carbonate, performing carbonization reaction, heating, performing thermal desorption to obtain lithium carbonate, performing solid-liquid separation to obtain high-quality lithium carbonate, and drying the high-quality lithium carbonate to obtain the battery-grade lithium carbonate. According to the method, carbonate and lithium in lithium extracted from lepidolite can be effectively recycled, the recycling cost is low, and the obtained lithium carbonate is high in purity and meets the requirements of battery-grade lithium carbonate.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium carbonate recovery, and in particular to a method for preparing battery-grade lithium carbonate by recovering lithium-extracted mother liquor from lepidolite. Background Art

[0002] With the rapid development of the global new energy industry and the continuous expansion of the electric vehicle market, the demand for lithium as a key raw material has shown explosive growth. At the same time, due to the endowment of lithium resources and the high processing costs, the price of lithium carbonate remains high, affecting the healthy development of the new energy industry. The main production process of lithium carbonate is to add sodium carbonate to lithium-containing brine extracted from raw materials such as lithium ore or salt lake to precipitate lithium, and then purify it to produce battery-grade lithium carbonate. Since lithium carbonate is a slightly soluble substance, the residual Li2O concentration in the lithium extraction mother liquor is 4g / L~7g / L, and the lithium extraction mother liquor needs to be further recovered and processed. The traditional process for treating the lithium extraction mother liquor generally undergoes decarbonization, evaporation, freezing and other processes to separate sodium, potassium and lithium salts, and the lithium-containing frozen liquid returns to the lithium precipitation process to further produce lithium carbonate. The problems with the traditional process are low ion utilization, especially low sodium carbonate utilization, which is only 60%~80%, and residual CO3 in the mother liquor. 2- As high as 15g / L-20g / L. Furthermore, the decarbonization process consumes large amounts of sulfuric acid and liquid caustic soda. It is estimated that the cost of recycling auxiliary materials from lithium extraction mother liquor to produce lithium carbonate is 6,000-7,000 yuan / ton, accounting for 40%-50% of direct processing costs. Therefore, developing a process that can effectively and cost-effectively recover ions from lithium extraction mother liquor has become a pressing issue for the lithium carbonate industry. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the object of the present invention is to provide a method for recovering battery-grade lithium carbonate from lithium mother liquor extracted from lepidolite. The method can effectively recover carbonate and lithium from lithium extracted from lepidolite, has low recovery cost, and the obtained lithium carbonate is high in purity, meeting the requirements of battery-grade lithium carbonate.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: A method for recovering battery-grade lithium carbonate from lithium-extracting mother liquor of lepidolite comprises the following steps: S1, evaporating and concentrating the lithium extraction mother liquor of lepidolite and precipitating a mixed salt to obtain a high-temperature evaporated slurry, and cooling the high-temperature evaporated slurry to obtain a low-temperature slurry; S2, introducing carbon dioxide into the low-temperature slurry for carbonization reaction to obtain carbonized slurry, and subjecting the carbonized slurry to solid-liquid separation to obtain a primary separation mother liquor and a potassium-sodium mixed salt; S3, cooling the primary separation mother liquor to precipitate sodium, and performing solid-liquid separation to obtain a frozen liquid and sodium sulfate decahydrate, heating the frozen liquid to precipitate lithium carbonate, and obtaining crude lithium carbonate and a primary pyrolysis mother liquor after solid-liquid separation; S4, slurrying the crude lithium carbonate to obtain a lithium carbonate slurry, introducing carbon dioxide into the lithium carbonate slurry for carbonization reaction to obtain a carbonized liquid, heating the carbonized liquid to pyrolyze lithium carbonate, and obtaining fine lithium carbonate and secondary pyrolysis mother liquor after solid-liquid separation; S5. Dry the fine lithium carbonate to obtain battery-grade lithium carbonate.

[0005] Furthermore, the concentrations of the main components of the lithium extraction mother solution of lepidolite in S1 are: Li2O concentration of 4g / L to 10g / L, potassium concentration of 1g / L to 80g / L, sodium concentration of 1g / L to 130g / L, CO3 2- The concentration is 1g / L~50g / L.

[0006] Furthermore, the concentrations of the main components of the lithium extraction mother solution of lepidolite in S1 are: Li2O concentration of 4g / L~7g / L, potassium concentration of 30g / L~60g / L, sodium concentration of 30g / L~70g / L, CO3 2- The concentration is 10 g / L to 40 g / L, and the pH value of the lithium extraction mother solution of lepidolite is 10 to 14.

[0007] Furthermore, the density of the high temperature evaporated slurry in S1 is 1 g / cm 3 ~2.0g / cm 3 The temperature of the high-temperature evaporated slurry is 50°C to 100°C; according to some preferred embodiments, the density of the high-temperature evaporated slurry in S1 is 1.4 g / cm 3 ~1.7g / cm 3 , the temperature of the high-temperature evaporation slurry is 85℃~100℃; Furthermore, the temperature of the low-temperature magma in S1 is 10° C. to 45° C. According to some preferred embodiments, the temperature of the low-temperature magma is 18° C. to 35° C.

[0008] Furthermore, the pressure of the carbonization reaction in S2 is 0.01 MPa to 0.3 MPa, the carbonization reaction time is 0.5 h to 5 h, and the pH of the slurry after carbonization is 8 to 10. According to some preferred embodiments, the pressure of the carbonization reaction in S2 is 0.05 MPa to 0.3 MPa, the carbonization time is 1 h to 5 h, and the pH of the slurry after carbonization is 8 to 9.2.

[0009] After the lithium mother liquor extracted from lepidolite is evaporated and concentrated to a certain density, it is cooled and a large amount of mixed salt will precipitate. Then, carbon dioxide is introduced into the low-temperature slurry to convert the lithium carbonate in the mixed salt into lithium bicarbonate with good solubility. The sodium-potassium mixed salt is separated through solid-liquid separation.

[0010] The sodium-potassium mixed salt is mainly composed of sodium sulfate and potassium sulfate, and contains a small amount of lithium. Furthermore, the sodium-potassium mixed salt described in S2 is recycled as a lepidolite roasting aid. When used as a roasting aid, it can be mixed according to a formula of lepidolite concentrate: calcium sulfate: calcium carbonate: potassium-sodium sulfate mixed salt = 63:15:10:12. The by-product sodium-potassium mixed salt can be reused and the lithium therein can be fully utilized.

[0011] Furthermore, the temperature of the cooling and sodium precipitation in S3 is -20°C to 5°C, and the Li2O concentration in the liquid after freezing is 12g / L to 30g / L. According to some preferred embodiments, the temperature of the cooling and sodium precipitation in S3 is -5 to 5°C, and the sodium salt can be further removed by the cooling and sodium precipitation process. The separated sodium sulfate decahydrate can be sold as a by-product after purification, and the secondary separation mother liquor obtained by dissolving the sodium sulfate decahydrate and evaporating and separating is returned to S3 and mixed with the separation mother liquor for reuse.

[0012] Furthermore, the temperature of the pyrolysis in S3 is 80°C to 100°C, and the pyrolysis time is 1h to 5h. According to some preferred embodiments, the temperature of the pyrolysis in S3 is 85°C to 100°C, and the pyrolysis time is 1h to 3h. Through the pyrolysis process, lithium bicarbonate is converted into lithium carbonate, and crude lithium carbonate can be obtained by solid-liquid separation.

[0013] The primary pyrolysis mother liquor obtained at this time contains a large amount of residual lithium, potassium, and sodium. Furthermore, the S3 primary pyrolysis mother liquor is returned to S1 and mixed with the lithium extraction mother liquor from lepidolite to enter the cycle. According to some preferred embodiments, the mixed volume ratio of the S3 primary pyrolysis mother liquor and the lithium extraction mother liquor from lepidolite is 1.5-2.5:10; Furthermore, the slurry preparation method in S4 is to prepare the crude lithium carbonate by slurrying with secondary pyrolysis mother liquor and / or water to obtain lithium carbonate slurry, wherein the liquid-to-solid ratio used in the slurry preparation is 20 to 30:1; Furthermore, the slurry preparation method in S4 is to prepare the slurry of the crude lithium carbonate by using a mixture of the secondary pyrolysis mother liquor and water to obtain a lithium carbonate slurry; Furthermore, the ratio of the secondary pyrolysis mother liquor to water is 10 to 1:1.

[0014] Furthermore, the pressure of the carbonization reaction in S4 is 0.001MPa~0.3MPa, the carbonization reaction time is 0.5h~5h, and the pH of the slurry after carbonization is 8~10. According to a further preferred embodiment, the pressure of the carbonization reaction in S4 is 0.003MPa~0.12MPa, the carbonization reaction time is 3h~5h, and the pH of the slurry after carbonization is 8.5~9.5. Furthermore, the temperature of the temperature-raising pyrolysis in S4 is 80℃~100℃, and the temperature-raising pyrolysis time is 1h~5h. According to a further preferred embodiment, the temperature of the temperature-raising pyrolysis in S4 is 85℃~95℃, and the temperature-raising pyrolysis time is 1.5h~3.5h.

[0015] Furthermore, the wet lithium carbonate is dried in an oven at a temperature of 220° C. to 280° C. for a time of 1 hour to 5 hours.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention provides a method for recovering the mother liquor of lithium extracted from lepidolite to prepare battery-grade lithium carbonate. The mother liquor of lithium extracted from lepidolite is used as raw material, and the mother liquor of lithium extracted is evaporated and concentrated to precipitate potassium and sodium salts and lithium carbonate. After cooling, carbon dioxide is introduced to dissolve the lithium carbonate in the slurry into lithium bicarbonate. For further pyrolysis and purification, the solubility of potassium and sodium salts decreases sharply under freezing and low-temperature conditions, and potassium and sodium salts are preferentially separated from the solution. The frozen liquid is then purified to produce battery-grade lithium carbonate. This method efficiently recovers the residual lithium and carbonate in the mother liquor of lithium extracted, and the produced lithium carbonate meets the battery-grade standard. In addition, the by-product potassium and sodium salts containing lithium can be used as a roasting aid, thereby improving the overall yield of lithium. In addition, this process only consumes low-cost carbon dioxide and water in the process of recovering battery-grade lithium carbonate, and does not require the use of additional recovery reagents. The primary separation mother liquor, secondary separation mother liquor, primary pyrolysis mother liquor and secondary pyrolysis mother liquor generated in the recovery process can all be returned to the system for reuse. No wastewater and solid waste are generated in the recycling process. The process is environmentally friendly and the production process is simple, which is easy to industrialize. It improves the yield of lithium extraction from lepidolite and reduces the production cost of lithium extraction from lepidolite. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a process flow chart for recovering lithium mother liquor from lepidolite to prepare battery-grade lithium carbonate in an embodiment of the present application. DETAILED DESCRIPTION

[0018] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. The embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0019] The main components of the lithium extraction mother solution of lepidolite in the following examples of this application are shown in Table 1 below: Table 1: Main components of lithium extraction mother liquor from lepidolite

[0020] Unless otherwise specified, the methods are conventional methods, and the raw materials can be obtained from public commercial channels unless otherwise specified.

[0021] Example 1 First-stage evaporation: 10m3 of potassium sulfate, sodium sulfate, and lithium carbonate in Table 1 3 The lithium mother liquor extracted from lepidolite is transferred to the evaporator for evaporation and concentration to precipitate the mixed salt. When the slurry density reaches 1.5g / cm 3 When the material is discharged, the final result is 4.35m 3 , high temperature evaporation slurry with a temperature of 92°C; Cooling: All high-temperature evaporated slurry is transferred to the cooler to cool down to 23°C to obtain low-temperature slurry; First-stage carbonization: transfer the low-temperature slurry into the carbonizer, start stirring, introduce carbon dioxide to maintain the pressure in the kettle at 0.25Mpa, and after carbonization for 1.5 hours, when the online pH meter shows the pH value is 8.46, release the pressure and discharge the material to obtain the carbonized slurry. The carbonized crystals will be separated by centrifuge to obtain 4.08m 3 The first-stage separation mother liquor has a Li2O concentration of 12.75 g / L, a K content of 40 g / L, and a Na content of 120 g / L. The weight of the potassium-sodium sulfate mixed salt is 1092 kg, with a Li2O content of 0.84%, a K content of 30.84%, and a Na content of 10.11%. Sodium precipitation by freezing: 4.08m 3 The mother liquor of the first stage separation was transferred to the freezing crystallizer for cooling and sodium precipitation. When the slurry temperature reached 0℃, it was transferred to the centrifuge for solid-liquid separation to obtain 2.12m 3 The frozen liquid has a Li2O concentration of 22.05g / L, a K concentration of 35g / L, and a Na concentration of 40g / L. 2832.68kg of sodium sulfate decahydrate was produced. 1034kg of potassium sodium sulfate mixed salt was obtained by further pulping, dissolving and evaporation of sodium sulfate decahydrate, with a Li2O content of 0.0051% and 0.94m sodium sulfate decahydrate evaporation mother liquor. 3 , where the Li2O concentration is 5.5g / L; Primary pyrolysis: 2.12m 3 After freezing, the liquid was transferred to a pyrolysis kettle, heated to 95°C, and kept warm for 2 hours to precipitate lithium carbonate. After solid-liquid separation, 95.56 kg of crude lithium carbonate was obtained, of which the main content was 97.76%, K content was 0.25%, Na content was 0.32%, SO4 2- The content is 1.16%. The volume of the first pyrolysis mother liquor is 1.978m3 , where the Li2O concentration is 4.57 g / L, the K concentration is 37.45 g / L, and the Na concentration is 42.78 g / L; Carbonization slurry preparation: 95.56 kg crude lithium carbonate is added with 2.1 ml deionized water at a liquid-to-solid ratio of 22:1. 3 ; Secondary carbonization: After the carbonization pulping is completed, it is transferred to the carbonizer and filled with carbon dioxide to maintain the pressure in the kettle at 0.08Mpa. After carbonization for 4 hours, when the online pH meter shows the pH value is 9.33, the pressure is released and the material is discharged to obtain 2.12m 3 The liquid after secondary carbonization has a Li2O concentration of 17.9 g / L; Secondary pyrolysis: After secondary carbonization, the liquid is heated to 90°C and kept warm for 1.8 hours to precipitate lithium carbonate. After solid-liquid separation, 75.9 kg of wet fine lithium carbonate and 2.08 m 3 Secondary pyrolysis mother liquor, where the Li2O concentration is 4.32 g / L; Drying: The wet fine lithium carbonate was transferred into an oven at a drying temperature of 250°C for 2 h to obtain 71.42 kg of battery-grade lithium carbonate. The product specifications are shown in Table 2.

[0022] Example 2 1.978m in Example 1 3 The mother liquor of primary pyrolysis and 10m 3 After the lithium extraction mother liquor is mixed, the method is used to further recover lithium carbonate.

[0023] First stage evaporation: 12.1m 3 The mother liquor is transferred to the evaporator for evaporation and concentration to precipitate the mixed salt. When the slurry density reaches 1.67g / cm 3 Start discharging when the material is discharged, and finally get 3.6m 3 , high temperature evaporation slurry with a temperature of 98°C; Cooling: All high-temperature evaporated slurry is transferred to the cooler to cool down to 32°C to obtain low-temperature slurry; First-stage carbonization: The low-temperature slurry is transferred to the carbonizer, stirring is started, and carbon dioxide is introduced to maintain the pressure in the kettle at 0.05Mpa. After carbonization for 5 hours, when the online pH meter shows a pH value of 9.02, the pressure is released and the material is discharged to obtain the carbonized slurry. The carbonized slurry is separated by a centrifuge to obtain 2.99m 3 The first-stage separation mother liquor has a Li2O concentration of 15.275 g / L, a K content of 40.3 g / L, and a Na content of 118.5 g / L. The weight of the potassium-sodium sulfate mixed salt is 2020.12 kg, with a Li2O content of 1.22%, a K content of 22.5%, and a Na content of 16.14%. Sodium precipitation by freezing: 2.99m 3The mother liquor of the first stage separation was transferred to the freezing crystallizer for cooling and sodium precipitation. When the slurry temperature reached 5°C, it was transferred to the centrifuge for solid-liquid separation to obtain 1.79m 3 The frozen liquid has a Li2O concentration of 22.91g / L, a K concentration of 35g / L, and a Na concentration of 40g / L. 2008.5kg of sodium sulfate decahydrate was produced. 634.65kg of potassium sodium sulfate mixed salt was obtained by further pulping, dissolving and evaporation of sodium sulfate decahydrate, with a Li2O content of 0.0072% and a sodium sulfate decahydrate evaporation mother liquor of 0.82m 3 , where the Li2O concentration is 5.3 g / L; Primary pyrolysis: 1.79m 3 After freezing, the liquid was transferred to a pyrolysis kettle, heated to 85°C, and kept warm for 1 hour to precipitate lithium carbonate. After solid-liquid separation, 81.25 kg of crude lithium carbonate was obtained, of which the main content was 98.21%, K content was 0.2%, Na content was 0.33%, SO4 2- The content is 1.33%. The volume of the mother liquor after the first pyrolysis is 1.778m 3 , where the Li2O concentration is 4.84 g / L, the K concentration is 35.21 g / L, and the Na concentration is 40.2 g / L; Carbonization slurry preparation: 81.25kg crude lithium carbonate at a liquid-to-solid ratio of 20:1, add 1.63ml deionized water 3 ; Secondary carbonization: After the carbonization pulping is completed, it is transferred to the carbonizer and filled with carbon dioxide to maintain the pressure in the kettle at 0.003Mpa. After carbonization for 5 hours, when the online pH meter shows the pH value is 8.57, the pressure is released and the material is discharged to obtain 1.66m 3 The liquid after secondary carbonization has a Li2O concentration of 19.57 g / L; Secondary pyrolysis: After secondary carbonization, the liquid is heated to 90°C and kept warm for 3 hours to precipitate lithium carbonate. After solid-liquid separation, 66.74 kg of wet fine lithium carbonate and 1.62 m 3 Secondary pyrolysis mother liquor, where the Li2O concentration is 4.38 g / L; Drying: The wet fine lithium carbonate was transferred into an oven at a drying temperature of 250°C for 2 h to obtain 63.08 kg of battery-grade lithium carbonate. The product specifications are shown in Table 2.

[0024] Example 3 2020.12 kg of potassium sodium sulfate (with a Li2O content of 1.22%) in Example 2 was recycled as a lepidolite roasting aid.

[0025] Mix lepidolite concentrate (1.976% Li2O content), calcium sulfate (15 kg), calcium carbonate (10 kg), and potassium sodium sulfate (12 kg) in a ratio of 63:15:10:12. Mix lepidolite concentrate (1.976% Li2O content), calcium sulfate (15 kg), calcium carbonate (10 kg), and potassium sodium sulfate (12 kg) and roast until smooth.

[0026] The mixed material was put into a sagger and placed in a high-temperature furnace. The temperature of the high-temperature furnace was adjusted to 1000°C. After keeping warm for 1 hour, the high-temperature furnace was closed. After cooling to room temperature, the mixed material was taken out and crushed, and 95.25 kg of clinker was obtained by weighing.

[0027] The obtained clinker was added with 95 L of water at a liquid-to-solid ratio of 1, stirred for 1 h, and then filtered to obtain 82.53 kg of leaching residue and 80.76 L of leachate. The metal balance data of this example are shown in Table 3.

[0028] Table 2: Main components of battery-grade lithium carbonate produced in Example

[0029] Table 3: Metal Balance Table of Example 3

[0030] The present invention's technology for recovering battery-grade lithium carbonate from lithium mother liquor precipitation eliminates the consumption of sodium carbonate, liquid caustic soda, and sulfuric acid, reducing production costs by 40-50%. The potassium and sodium salts produced by this method, which carry lithium, are further recycled as a calcination aid for lepidolite, thereby improving lithium recovery. This method not only improves the lithium recovery rate of lepidolite mother liquor, but also effectively recovers carbonate ions from the mother liquor. The resulting lithium carbonate product meets battery-grade standards (YS / T582-2013). This method features a simple production process, low recovery costs, and ease of industrial production, significantly improving the utilization of carbonate and lithium from the mother liquor.

[0031] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them. Although the present application has been described in detail with reference to preferred embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present application can still be modified or some technical features can be replaced by equivalents, which should all be included in the scope of the technical solutions requested for protection in this application.

Claims

1. A method for recovering battery-grade lithium carbonate from lithium mica mother liquor, characterized in that: The following steps are involved: S1, evaporating and concentrating the lithium extraction mother liquor of lepidolite and precipitating a mixed salt to obtain a high-temperature evaporated slurry, and cooling the high-temperature evaporated slurry to obtain a low-temperature slurry; S2, introducing carbon dioxide into the low-temperature slurry for carbonization reaction to obtain carbonized slurry, and subjecting the carbonized slurry to solid-liquid separation to obtain a primary separation mother liquor and a potassium-sodium mixed salt; S3, cooling the primary separation mother liquor to precipitate sodium, and performing solid-liquid separation to obtain a frozen liquid and sodium sulfate decahydrate, heating the frozen liquid to precipitate lithium carbonate, and obtaining crude lithium carbonate and a primary pyrolysis mother liquor after solid-liquid separation; S4, slurrying the crude lithium carbonate to obtain a lithium carbonate slurry, introducing carbon dioxide into the lithium carbonate slurry for carbonization reaction to obtain a carbonized liquid, heating the carbonized liquid to pyrolyze lithium carbonate, and obtaining fine lithium carbonate and secondary pyrolysis mother liquor after solid-liquid separation; S5. Dry the fine lithium carbonate to obtain battery-grade lithium carbonate.

2. A method for recovering battery-grade lithium carbonate from lithium mica mother liquor according to claim 1, characterized in that: The concentrations of the main components of the lithium extraction mother solution of lepidolite in S1 are: Li2O concentration of 4g / L~10g / L, potassium concentration of 1g / L~80g / L, sodium concentration of 1g / L~130g / L, CO3 2- The concentration is 1g / L~50g / L. Preferably, the concentrations of the main components of the lithium mother solution extracted from lepidolite in S1 are: Li2O concentration of 4g / L to 7g / L, potassium concentration of 30g / L to 60g / L, sodium concentration of 30g / L to 70g / L, CO3 2- The concentration is 10 g / L to 40 g / L, and the pH value of the lithium extraction mother solution of lepidolite is 10 to 14.

3. A method for recovering battery-grade lithium carbonate from a lithium mica mother liquor according to claim 1, characterized in that: The density of the high temperature evaporated slurry in S1 is 1g / cm 3 ~2.0g / cm 3 , the temperature of the high-temperature evaporation slurry is 50℃~100℃; And / or, the temperature of the low-temperature slurry is 10°C to 45°C.

4. A method for recovering battery-grade lithium carbonate from a lithium mica mother liquor according to claim 1, characterized in that: The pressure of the carbonization reaction in S2 is 0.01 MPa to 0.3 MPa, the carbonization reaction time is 0.5 h to 5 h, and the pH of the slurry after carbonization is 8 to 10.

5. A method for recovering battery-grade lithium carbonate from a lithium mica mother liquor according to claim 1, characterized in that: The sodium-potassium mixed salt described in S2 is used as a calcination aid for lepidolite.

6. The method for recovering battery-grade lithium carbonate from a lithium mica mother liquor according to claim 1, wherein: The temperature of the cooling and sodium precipitation in S3 is -20°C to 5°C, and the Li2O concentration in the liquid after freezing is 12g / L to 30g / L.

7. The method for recovering battery-grade lithium carbonate from lithium mica mother liquor according to claim 1, wherein: The temperature of the pyrolysis in step S3 is 80°C to 100°C, and the pyrolysis time is 1h to 5h; And / or, the mother liquor from the primary pyrolysis of S3 is returned to S1 and mixed with the mother liquor from lithium extraction from lepidolite.

8. The method for recovering battery-grade lithium carbonate from lithium mica mother liquor according to claim 1, wherein: The slurry preparation method in S4 is to prepare the crude lithium carbonate by slurrying with secondary pyrolysis mother liquor and / or water to obtain lithium carbonate slurry, wherein the liquid-to-solid ratio used in the slurry preparation is 20 to 30:1; Preferably, the slurry preparation method in S4 is to prepare the slurry of the crude lithium carbonate by using a mixture of the secondary pyrolysis mother liquor and water to obtain a lithium carbonate slurry; Preferably, the ratio of the secondary pyrolysis mother liquor to water is 10 to 1:

1.

9. A method for recovering battery-grade lithium carbonate from lithium mica mother liquor according to claim 1, characterized in that: The pressure of the carbonization reaction in S4 is 0.001 MPa to 0.3 MPa, the carbonization reaction time is 0.5 h to 5 h, and the pH of the slurry after carbonization is 8 to 10.

10. The method for recovering battery-grade lithium carbonate from lithium mica mother liquor according to claim 1, wherein: The temperature of the pyrolysis in S4 is 80° C. to 100° C., and the pyrolysis time is 1 hour to 5 hours.

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